CFU measurments - (Apr/25/2013 )

I have done CFU measurement of E.coli at OD~0.3. I am getting different results as describe below:

First time I did the experiments the CFU results showed that 0.3 OD = 2.07 x 108 cells/ml.

After some weeks I did the same experiment and the CFU results are showed to me that 0.3 OD = 1.41 x 109 cells/ml.

I am not sure why I am getting that different values of CFU for same OD?

Any help will be highly appreciated. Thanks

-Fiaq Khan-

Fiaq Khan on Thu Apr 25 12:32:23 2013 said:

I have done CFU measurement of E.coli at OD~0.3. I am getting different results as describe below:

First time I did the experiments the CFU results showed that 0.3 OD = 2.07 x 108 cells/ml.

After some weeks I did the same experiment and the CFU results are showed to me that 0.3 OD = 1.41 x 109 cells/ml.

I am not sure why I am getting that different values of CFU for same OD?

Any help will be highly appreciated. Thanks

Did you do everything the same way? Same protocol?
It can be because of so many things... Not the same amount of cells at the start... not the same time of incubation? Less or more aeration....
Also: in the end the difference is not that big...

-pito-

I agree there could be a ton of reasons why: viability caused by incubation time, media formulation, incubation temp, shaker speed....

A log difference in cfu is rather significant to me.

-Paulgs3-

Paulgs3 on Fri Apr 26 12:26:58 2013 said:

I agree there could be a ton of reasons why: viability caused by incubation time, media formulation, incubation temp, shaker speed....

A log difference in cfu is rather significant to me.

if the samples where the same yes.
But here: since we dont know what might have gone "wrong" or different it means pretty much nothing.
Maybe I was not clear enough about that.

-pito-

I have done everything same. Same bacteria, same media and same OD which is 0.3. Is that difference in CFU is acceptable? or how much variation is possible?

btw - did you generate a standard curve that includes these bacterial titers - or were these just 2 isolated observations? You should ensure that you can (by standard curve) differentiate between such high concentrations of cells.